In the field of magnetic recording media having a magnetic layer consisting mainly of ferromagnetic particles and a binder resin, there is a growing demand for higher--density recording. Smoothing the surfaces of the magnetic layers is one of the known measures to satisfy such demand.
However, the enhanced surface smoothness of the magnetic layer causes the coefficient of friction between the magnetic layer of the magnetic recording medium being running and the recording or reproducing device's parts in contact with the magnetic layer to be so heightened that the magnetic layer is apt to suffer damages or peel off in a relatively short period of use.
Floppy disks, in particular, are often used under severe high-temperature conditions as in durability tests. In such cases, the magnetic layers are occasionally apt to shed ferromagnetic particles, which may have caused clogging of magnetic heads.
Further, as an effective means for obtaining magnetic recording media that can record information at high densities, it is known to employ ferromagnetic metal particles as the ferromagnetic particles.
The use of ferromagnetic metal particles in a floppy disk, however, has been defective in that the magnetic layer is prone to suffer damages due to the rubbing contact of the magnetic layer with the liner in the jacket, and this may cause dropouts. This problem has often been encountered particularly when floppy disks are used under conditions of relatively high temperatures such as 50 to 60.degree. C.
For improving the running durability of magnetic layers, it has been proposed to incorporate an abrasive agent (hard particles), such as corundum, silicon carbide, or chromium oxide, in magnetic layers. However, this technique is disadvantageous in that in order to fully produce the effect of improving the running durability of magnetic layers, the abrasive materials must be incorporated into the magnetic layers in considerably large amounts. Further, the incorporation of an abrasive agent has been insufficient in protecting the magnetic layer from being damaged by the liner.
In addition, the magnetic layers containing large proportions of abrasive agents cause excessive magnetic head wear etc. Furthermore, the incorporation of abrasive agents has been not preferred in that it is contrary to the way of improving electromagnetic characteristics by smoothing magnetic layers.
It has also been proposed to incorporate in magnetic layers a fatty acid or an ester of a fatty acid and an aliphatic alcohol, as a lubricant, so as to minimize the coefficient of friction.
For example, U.S. Pat. No. 3,833,412 proposes to incorporate a monobasic fatty acid having 4 carbon atoms or more, such as caprylic acid, lauric acid, or oleic acid, as a lubricant; U.S. Pat. No. 3,274,111 proposes to incorporate as a lubricant a fatty acid ester which is an ester of a monobasic fatty acid having 12 to 16 carbon atoms and an aliphatic alcohol having 3 to 12 carbon atoms, examples of such ester including butyl laurylate, lauryl palmitate, and butyl myristate; and U.S. Pat. No. 4,303,738 proposes to incorporate tridecyl stearate as a lubricant.
JP-A-55-157131 proposes to incorporate as a lubricant an ester of a higher fatty acid and a higher alcohol having a branched molecular structure, and also JP-A-59-186130 proposes to incorporate isocetyl stearate as a lubricant. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, the incorporation of such lubricants adversely affects the strength of the magnetic layers, although the incorporation is effective in lowering the coefficient of friction. Thus, the incorporation of lubricants has been insufficient in the effect of preventing the magnetic layers from damage caused by contact with the liners.
Moreover, use of cellulosic resins, as binder resins, having the high modulus of elasticity and the excellent mechanical strength so as to improve the liner-abrasion resistance of magnetic layers is disclosed in, for example, JP-A-56-13519, JP-A-62-239316, JP-A-58-70424, JP- A-58-70425, JP-A-56-74833, JP-A-60-133527, JP-A-62-34326, JP-A-62-26628, JP-A-59-79428, JP-A-57-135439, JP-A-56-74832, and JP-A-59-188827.
Binder resins based on such conventional cellulosic resins including nitrocellulose, however, are insufficient in the ability to disperse ferromagnetic particles, so that not only the resulting magnetic layers cannot have a good surface property, but the packing density of ferromagnetic particles cannot be so heightened. Consequently, the use of conventional cellulosic resins has been ineffective in provision of magnetic recording media having excellent electromagnetic characteristics.
For attaining improved dispersion of ferromagnetic particles, incorporation of a carboxyl group in nitrocellulose has been proposed in, for example, JP-A-61-168120, JP-A-63-99276, JP-A-63-99271, and JP-B-63-14753. (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, such nitrocellulose has been unable to produce a sufficient dispersing effect when the ferromagnetic particles to be dispersed are very fine metal particles having a specific surface area of, for example, 40 m.sup.2 /g.